Recently, the development of optical disc apparatus capable of recording and reproducing information signals such as video or voice signal has been active. In the optical disc apparatus capable of recording, a guide groove has previously formed on an optical disc substrate to form tracks. Hereinafter, the track formed by the guide groove is referred to as a hollow portion and the track formed by a portion lies between guide grooves is referred to as a convex portion. Recording or reproduction of the information signal is effected by focusing laser light on a flat portion of the hollow portion or the convex portion of these tracks. In the optical disc, a phase change type material or the like is used, a reflectivity of a recording film thereof changing. In general optical disc apparatus on the market, generally, an information signal is recorded on either of the hollow or convex portion and the other is provided as a guide band for separating adjacent tracks.
FIG. 14 is an enlarged perspective view of an optical disc used in such a prior art optical disc apparatus. In FIG. 14, numeral 1 is a recording layer formed of a phase change material for example. Numeral 2 denotes a recording mark. Numeral 3 is a focused spot of laser light. Numeral 4 is a hollow portion and numeral 5 is a convex portion having a width larger than the hollow portion 4. Numeral 6 denotes an address pit indicative of positional information on the disc. Here, the recording mark is a region whose reflectivity changes by application of laser light having a predetermined intensity. This records the information signal. Moreover, the address pit has been formed as hollow and convex portions of the substrate of the disc previously in the production processing of the optical disc. In FIG. 14, a transparent disc substrate through which incident light transmits is omitted.
A prior art optical disc apparatus using this optical disc will be described with reference to drawings.
FIG. 15 is a block diagram of such a prior art optical disc apparatus. In FIG. 15, numeral 7 is an optical disc and numeral 8 is a recording track which is a convex portion 5 here. Numeral 10 is a semiconductor laser, numeral 11 is a collimation lens for collimating the laser light emitted by the semiconductor laser 10 to output a collimated light, numeral 12 is a half mirror located on a light beam, and numeral 13 is an objective lens for focusing the collimated light transmitted through the half mirror 12 on a recording plane on the optical disc 7. Numeral 14 is a photodetector for receiving a reflection light from the optical disc 7 via the object lens 13 and the half mirror 12 and has two photodetection portions 14a and 14b separated in a direction parallel to the track direction of the disc to obtain a tracking error signal. Numeral 15 is an actuator for supporting the object lens 13. The above-mentioned parts are mounted on a head base not shown and form an optical head 16. Numeral 17 is a differential amplifier supplied with detection signals outputted by the photodetection portions 14a and 14b, and numeral 18 is a lowpass filter (LPF) supplied with a differential signal outputted by the differential amplifier 17. Numeral 19 is a tracking control circuit supplied with an output signal of the LPF 18 for supplying a drive current to the actuator 15. Numeral 20 is a summing amplifier supplied with the detection signals outputted by the photodetection portions 14a and 14b for outputting a summed signal. Numeral 21 is a highpass filter (HPF) supplied with the summed signal from the summing amplifier 20 for supplying a high frequency component to a reproducing signal processing circuit 22 mentioned later, and numeral 22 is the reproduction signal processing circuit supplied with the high frequency component of the summed signal from, the HPF 21 for supplying an information signal such as a voice to an output terminal 23. Numeral 24 is an address reproduction circuit supplied with the high frequency component from the HPF 21 for supplying an address signal to a system controller 31 mentioned later. Numeral 25 is a traverse control circuit for supplying a drive current to a traverse motor 26 mentioned later in accordance with a control signal from the system controller 31 mentioned later. Numeral 26 is a traverse motor for moving the optical head 16 in a radial direction of the optical disc 7. Numeral 27 is a spindle motor for rotating the optical disc. Numeral 28 is a recording signal processing circuit supplied with the information signal such as a voice inputted through an external input terminal 29 for supplying the recording signal to an LD drive circuit 30 mentioned later. Numeral 30 is an LD drive circuit supplied with the recording current from the recording signal processing circuit 28 for inputting the drive current to the semiconductor laser 10. Numeral 31 is a system controller supplied with the address signal from the address reproduction circuit 24 for supplying a control signal to the traverse control circuit 25 and the recording signal processing circuit 28.
An operation of the prior art optical disc apparatus structured as mentioned above will be described with reference to drawing.
A laser beam emitted form the semiconductor laser 10 is converted into a collimated light beam by the collimation lens 11 and focused on the optical disc 7 by the objective lens 13 through the beam splitter 12. The light beam reflected by the optical disc 7 has information on the recording track 8 by diffraction and is introduced on the photodetector 14 by the beam splitter 12 via the object lens 13. The photodetection portions 14a and 14b convert changes in light amount distributions of the incident light beams into electric signals and supply them to the differential amplifier 17 and the summing amplifier 20. The differential amplifier 17 effects I-V conversion of respective input currents and obtains a difference between them to outputs a push-pull signal. The LPF 18 extracts a low frequency component from the push-pull signal to supply it to the tracking control circuit 19 as a tracking error signal. The tracking control circuit 19 flows a drive current through the actuator 15 to effect a position controlling of the object lens 13. This causes the focus spot to scan on the convex portion 5 correctly.
On the other hand, the summing amplifier 20 effects the I-V conversion of output currents of the photodetection portions 14a and 14b and sums them to supply it as the summed signal to the HPF21. The HPF 21 cuts off unnecessary low frequency components from the summed signal and passes the reproduced signal and the address signal as the main information signals to supply them to the reproduction processing circuit 22. The reproduction signal processing circuit 22 demodulates the reproduction signal inputted thereinto and then, effects an error correction processing or the like to supply a voice signal or the like to the output terminal 23. The address reproduction circuit 24 demodulates the address signal inputted thereinto and supplies it as position information on the disc to the system controller 31. That is, as the result of scanning of the focused spot 3 on the recording mark 2, the reproduction signal is inputted into the reproduction signal processing circuit 22. As the result of scanning on the address pit 6, the address signal is inputted to the address reproduction circuit 24. The system controller 31 supplies the control signal in accordance with the address signal to the traverse control circuit 25 to shift the optical head 16 to a desired position.
The traverse circuit 25 supplies a drive current to the traverse motor 26 in accordance with the control signal from the system controller 31 when the optical head is shifted in order to move the optical head 16 to a target track. At this instance, the tracking control circuit 19 temporally intercepts the tracking servo in accordance of the control signal from the system controller 31. Moreover, when the normal reproducing, it drives the traverse motor 26 in accordance with the low frequency component of the tracking error signal inputted from the tracking control circuit 19 to gradually move the optical head in the radial direction as the reproducing advances.
The recording signal processing circuit 28 adds error correction codes or the like to the voice signal or the like inputted from the external input terminal 29 when the recording and supplies it as a coded recording signal to the LD drive circuit 30. The LD drive circuit 30 modulates a drive current applied to the semiconductor laser 10 in accordance with the recording signal. This causes the light spot applied onto the optical disc 7 to have an intensity change in accordance with the recording signal to form recording marks 2.
During the respective operations mentioned above, the spindle motor 27 rotates the optical disc 7 at a constant linear or angular velocity.
Here, an interval between tracks was shortened by narrowing the a width of the hollow portion 4 to increase a recording capacity of the optical disc 7. However, there is a problem that the tracking error signal for controlling the focus spot 3 with a high accuracy decreases because a diffraction angle of the reflection light by the hollow portion 4 becomes Large if the track interval is narrowed. Moreover, there is a limit in narrowing the track interval by only width of the hollow portion 4, so that the width of the convex portion 5 should be narrowed. This will cause a problem of decrease in an amplitude of the reproduction signal because the mark 2 becomes thin.
On the other hand, as described in Japanese patent publication No. 63-57859, there is a technique that a track density is increased by recording information signals on both hollow potion 4 and convex portion 5.
FIG. 16 is an enlarged perspective view of such an optical disc. In FIG. 16, numeral 1 is a recording layer formed with a phase change material for example. Numeral 2 is a recording mark, and numeral 3 is a focus spot of laser light. The same or corresponding parts or element described with re-ference to FIG. 14 are designated as the same references. Numeral 40 is a hollow portion and numeral 41 is a convex portion. As shown in FIG. 16, the hollow portion 40 has approximately the same width as the convex portion 41.
In this optical disc, the recording marks 2 are formed on both hollow potions 40 and convex portions 41. The convex portion 41 has the same period as the convex portion 5 of the optical disc shown in FIG. 14. An interval of mark train is a half of that of the optical disc shown in FIG. 14. Hereinafter, both hollow portion 40 and convex portion 41 are referred to as recording tracks in the meaning that the recording marks 2 are formed. An operation of the recording/reproducing in the optical disc apparatus for this optical disc is carried out in the similar manner to the optical disc apparatus shown in FIG. 15 basically, However, it is necessary to invert a polarity of a tracking error signal between when the focus spot 3 scans on the convex portion 41 and when it scans on the hollow portion 40 as described in the Japanese patent publication No. 63-57859. This can be provided by inserting an inverting amplifier capable of the ON/OFF controlling between the LPF 18 and the tracking control circuit in FIG. 15.
On the other hand, there is a problem that in the technique disclosed in the Japanese patent publication No. 63-57859, it is impossible to effect the tracking of the focus spot on a target recording track with a high accuracy if either of recording tracks lie on the both side of the target recording track has been recorded and the other has not recorded. FIG. 17 is an enlarged perspective view of an optical disc in such a case. It shows that the recording marks 2 have been recorded on the recording track on the left side off the recording track on which the focus spot 3 scans but on the right side of the recording track there is nothing recorded. In the above-mentioned technique, since a recording track pitch is substantially half of a diameter of the focus spot, the focus spot overlaps the recording tracks adjacent to the recording track which is desired to be scanned. Therefore, the reflected light is effected by the mark train on the recording track adjacent thereto. The case as shown in FIG. 17 occurs if an optical disc on which nothing has been recorded over a whole surface is successively recorded from the inside circumference side. In this case, the recording track on an inner circumference side of the recording track under scanning has been recorded already and on the recording track outside of the recording track under scanning has not been recorded. Here, the reason why the accuracy of the tracking decreases is described in detail hereinbelow.
FIG. 18 shows an enlarged view (a) of a disc surface, an illustration (b) for showing brightness/darkness of the reflection light reaching the photodetector 14, and a cross-sectional view (c) of intensity distribution of the reflection light reaching the photodetector 14 in the case that the recording tracks on the both sides of the recording track on which the focus spot scans. However, these drawings show the light amount distribution of the reflection light approximately and is not accurate. Moreover, for convenience for explaining, they show the case that there is no recording marks on the center convex portion. As shown in FIG. 18(a), in the case that the focus spot scans at just center of the target recording track and the recording has not been effected on the recording tracks on the both sides thereof, the light amount distribution of the reflection light reaching the photodetector 14 is symmetric in the direction corresponding to the track direction as shown in FIG. 18(b) by the diffraction effect due to a difference in level between the hollow and convex portions. The light amount distribution in the cross section taken on the line A-B is shown in FIG. 18(c). The amounts of light received by the two photodetection portions 14a and 14b in the photodetector 14are equal each other. Therefore, a difference between levels of the detection signals outputted by the two photodetection portions 14 a and 14b through photoelectric conversion, that is, a DC level of the tracking error signal is zero.
FIG. 19 shows an enlarged view (a) of a disc surface, an illustration for showing brightness/darkness of the reflection light reaching the photodetector 14, and a cross-sectional view (c) of the intensity distribution of the reflection light reaching the photodetector 14 in the case that the recording has carried out on either of recording tracks adjacent to the recording track on which the focus spot scans. As shown (a) in FIG. 19, if the focus spot scans just the center of the target recording track and a portion of the focus spot overlaps a recording mark on the recording track on the inner circumference side, the light amount distribution of the reflection light reaching the photodetector 14 is asymmetric between right and left side as shown by (b) in FIG. 19 due to effect of the recording mark. The light amount distribution on the cross section taken on the line AB shown by (b) in FIG. 19 is as shown by (c) in FIG. 19. That is, the amounts of light received by the two photodetection portions 14a and 14b of the photodetector 14 are not equal each other. Therefore, the DC level of the tracking error signal is not zero (hereinafter the DC level in this case is assumed as Voff). That is, an offset of Voff occurs in the tracking error signal. Actually, a positional relation between the focus spot and the recording mark is such that the focus spot may not almost overlap any of the recording marks in the case that the focus spot locates between recording marks on the adjacent track as shown in FIG. 20. In this case, the DC offset in the tracking error signal obtained by the reflection light does not occur almost. Accordingly, a signal component of the data signal is mixed with the tracking error signal in accordance with the positional relation between the recording marks on the adjacent recording track and the focus spot. FIG. 21 shows a waveform of the tracking error signal with which the information signal is mixed. A component of the band of the information signal is removed from the tracking error signal by the LPF 18 as shown in FIG. 15, so that that tracking error signal has a value averaged between Voff and zero, that is, it has a DC offset. It is clear that if an optical system of the optical head 16 has no aberration and a cross section shape of the guide groove of the optical disc is symmetric with respect to the center of the groove, the DC offset has an equal absolute value and opposite polarities between the case that the recording mark exists on the hollow portion on the right and the case the recording mark exists on the hollow portion on the left as shown in FIG. 10.
Moreover, FIG. 22 shows an enlarged view (a) of a disc surface, an illustration for showing brightness/darkness of the reflection light reaching the photodetector 14, and a cross-sectional view (c) of the intensity distribution of the reflection light reaching the photodetector 14 in the case that the recording has been carried out on both recording tracks adjacent to the recording track on which the focus spot is scanning. As shown (a) in FIG. 22, if the focus spot scans just the center of the target recording track and a portion of the focus spot overlaps any of the recording marks on the adjacent recording tracks on the both sides, the light amount distribution of the reflection light reaching the photodetector 14 is symmetric between right and left sides with respect to the track direction as shown by (b) in FIG. 22 due to effect of the recording mark on both sides. The light amount distribution on the cross section taken on the line AB shown by(b) in FIG. 22 is as shown by (c) in FIG. 22. That is, the amounts of light received by the two photodetection portions 14a and 14b on the photodetector 14 are equal each other. Therefore, the DC level of the tracking error signal is zero. However, a DC level occurs in the tracking error signal in accordance with relative positional relation between the recording marks on the recording track on the both sides in the case also. However, since the DC levels have both positive and negative polarities, it can be easily predicted that the DC levels become zero by averaging by the LPF 18.
As mentioned above in detail, there is a problem that the offset occurs in the tracking error signal if the recording marks exist only on the either of the recording tracks adjacent to the target recording track to be scanned by the focus spot, so that the tracking controlling becomes inaccurate.